1. Physiological and pharmacological properties of the human neuronal alpha4beta2 nicotinic AChR and mutants found in patients suffering from autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) were studied. 2. Investigations of nicotinic AChRs reconstituted in Xenopus oocytes with the control or mutated alpha4 subunits revealed that both mutation S248F as well as the Leucine insertion (776ins3) result in major but different changes in the physiological and pharmacological properties of the receptors. 3. Mutation S248F causes a decrease in apparent affinity to ACh of about 7 fold. In addition, this receptor already desensitizes during exposure to agonist concentration 3000 times lower than the control. 4. 776ins3 provokes a 10 fold increase of apparent ACh affinity, an increase in the IC50 caused by prolonged ACh exposures and a slowing down of the response decay. 5. At saturating ACh concentration cells expressing the S248F mutant display average currents that are about five times smaller than control. 6. When measured at very low concentration, agonist sensitivities of the control and mutated receptors to ACh, nicotine and epibatidine exhibit differences that match those observed for higher agonist concentrations. 7. Mutation 776ins3 increases the apparent efficacy to cytisine. 8. Data presented herein suggest that mutation S248F mainly affects the desensitization properties of the receptor while the leucine insertion (776ins3) increases the probability of transition to the active state. Although these mutations differentially affect the receptor properties they both result in reduced permeability to calcium and enhanced desensitization sensitivity that might account for the ADNFLE phenotype.